Electrical wire is used to transmit electronic signals. The wire must be protected, or insulated, and plastic coatings are ordinarily extruded from a molten state onto and around the wire. These plastic materials are chosen to have a low dielectric constant and a low dissipation factor. It has previously been found that if the plastic is foamed as it is applied to the wire, the dielectric constant is desirably lowered, due to the formation of numerous small cells in the foam.
Foamed insulation around transmission wire is described in U.S. Pat. No. 3,072,583 to S. K. Randa where he describes a nucleated foaming process for extruding perfluorinated polymer, e.g., ethylene/propylene (FEP) copolymer, foam with a dissolved gas blowing agent. Because of its high viscosity, FEP foam is difficult to extrude onto wire at high rates.
U.S. Pat. Nos. 4,331,619 and 4,394,460 to Chung et al. relate to a nucleated, chemically blown foam composition based on fluorinated copolymers, preferably ethylene-chlorotrifluoroethylene copolymers. This patent describes the foam on wire only in terms of average cell size. It does not address the problem of low dielectric strength due to structural failure of the foam. Foam insulation can be weakened structurally and electrically when several foam cells are aligned radially between the wire and the outer surface of the foam insulation, and/or when two or more foam cells substantially larger than average are so aligned, or when the size of a single cell approaches the thickness of the insulation. Electrical test data are not reported in these patents, but the Examples imply that pin holes are present in the coating, by stating that "a minimum of pinholes appear at the surface".
Sometimes a skin or jacket is placed around the foam wire construction to protect the assembly. For example, U.S. Pat. No. 4,309,160 to Poutanen et al. discloses an apparatus and a method for forming a foam and an unfoamed skin around telephone wire. The patent points out that the foam provides good electrical properties (i.e., low dielectric constant) and the unfoamed outer layer, or skin, provides good mechanical properties. In this patent, the foam and skin are made of the same plastic material, but fluorinated polymers are not mentioned.
Similarly, Maillefer Technical Report 17 teaches extrusion techniques for foam-skin extrusion onto wire. It deals mainly with polyethylene foam/polyethylene skin constructions, and does not suggest the use of fluorine-containing polymers.
There is a need for high speed extrusion of both foam and skin onto electrical wire.